Biopotentials are usually recorded using electrodes attached to the body, such as wet (gel) electrodes, or dry electrodes. The electrodes are used to measure biopotentials, which typically have a magnitude in the range of about 50 uV to 10 mV.
The electrodes can have different polarization voltages, resulting in a large DC or very slowly varying signal (time constant well below 1 s) present between the electrodes, for example, on the order of 300 mV. Furthermore, the biopotential signals can be affected by interference currents derived from mains power supply lines, known as “common mode aggressors.” The mains frequency generally falls within the frequency range of interest of biomedical signals. For example, an ECG signal has its main frequency components in a range between about 0.5 Hz to 40 Hz, but signal information up to around 200 Hz may be desired.
Instrumentation amplifiers (“IAs”) are generally used for biomedical signals, as result of their high common mode rejection ratio, enabling small differential signals to be amplified with a large gain. However, a large DC offset can saturate the IA.
To address this potential problem, the IA is for example preceded by a high-pass filter. This filter may use at least two large external capacitors to implement sufficiently low cut-off frequency without impacting important performance metrics.
Alternatively, a DC-coupled architecture is used. To avoid channel saturation in this case, the IA may not have too much gain, which means that the requirements in terms of noise and dynamic range for subsequent blocks (which can include programmable gain amplifiers, filters and analog-to-digital converters) become stricter.
A DC-coupled architecture typically uses a resistive feedback loop. A possible drawback of this approach is that only low gain factors can be used to avoid channel saturation. As a consequence, there may be strict requirements in terms of noise and dynamic range on the subsequent processing blocks.
One article that generally discloses the use of an analog feedback loop to provide DC-offset cancellation is: Denison T., Consoer K., Kelly A., Hachenburg A., Santa W., “A 2.2 μW 94 nV/√Hz, Chopper-Stabilized Instrumentation Amplifier for EEG Detection in Chronic Implants,” IEEE International Solid-State Circuits Conference, pp. 162-594, 11-15 Feb. 2007.